With the spread of the Internet, an increase in the capacity of an optical communication system has progressed. For example, in trunk systems, researches for transmitting signals at a rate exceeding 40 Gbit/s per wavelength have been performed. When a bit rate per wavelength is made high, the deterioration of signal quality increases due to a decrease in optical signal-to-noise ratio (OSNR) resistance, and waveform distortion caused by wavelength dispersion of a transmission channel, polarization mode dispersion, a non-linear effect, and the like.
For this reason, in recent years, digital coherent reception systems having high OSNR resistance and waveform distortion resistance have attracted attention (see, for example, Patent Documents 1 and 2). In the digital coherent reception systems, light intensity information and phase information are extracted from a received signal, and demodulation is performed by a digital signal processing circuit. In the digital coherent reception systems, an improvement in the OSNR resistance by coherent reception and a correction of the waveform distortion by the digital signal processing circuit are realized, and thus even when signals are transmitted at a rate exceeding 40 Gbit/s, high reliability is achieved.